The present invention relates to pay television channel selectors, and more particularly to a pay television channel selector for CATV or the like in which a channel broadcast signal selected on the basis of a reception contract is received, and the tapping of an uncontracted channel signal is prevented.
In buildings such as apartment houses and the like, it is difficult to install antennas in the respective rooms. It is therefore common practice to install an antenna on the rooftop and to distribute signals received by the antenna to TV receivers in the respective rooms through cables. Further, in regions where reception of broadcast TV signals is difficult, cable systems have been provided to contractors or subscribers in order to distribute television signals to the viewers through transmission cables. In such CATV systems, pay television channel selectors through which only the signals of contracted channels can be viewed are typically provided. They are constructed so as to inhibit the contractors from viewing the signals of uncontracted channels.
A known pay television channel selector typical of the prior art will be described with reference to FIG. 1. In the figure, numeral 1 designates a reception portion for selecting channels which have been properly contracted for 2-A1 and 2-B1 indicate connecting terminals of Channel 1 (CH1) for a jumper lead, and 2-A2 and 2-B2 jumper lead terminals of Channel 2 (CH2). Similarly, 2-An and 2-Bn represent jumper lead terminals of Channel n (CHn). 3-1 indicates a jumper lead. In the example of FIG. 1, Channel 1 has been contracted for, and the jumper lead 3-1 is connected between the jumper lead terminals 2-A1 and 2-B1. In this manner, when Channel n is contracted, the jumper lead terminals 2-An and 2-Bn of Channel n (CHn) are short-circuited by the jumper lead 3-n. 4-1 to 4-n indicate pay channel switches, which interlock with respectively corresponding channel-selecting switches to be described later. Numeral 5 designates a pay control output terminal, numeral 6 a resistor, numeral 7 a power supply terminal, and numeral 8 a channel selecting portion. 9-1 to 9-n designate potentiometers for generating tuning voltages. The channel selecting switches 10-1 to 10-n are paired with the aforecited channel pay switches 4-1 to 4-n and perform interlocking operations therewith, respectively. Numeral 11 indicates a tuning voltage supply input terminal which is fed with a predetermined supply voltage, while numeral 12 indicates a tuning voltage output terminal which delivers as the tuning voltage of a receiving set a voltage produced in the tuning voltage generating potentiometer of the selected channel.
The operation of the pay television channel selector in FIG. 1 will be briefly described. In case where reception of Channel 1 (CH1) has been contracted for, the jumper lead 3-1 is connected between the jumper lead connecting terminals 2-A1 and 2-B1 of Channel 1 as shown in FIG. 1. Accordingly, when the contractor or subscriber selects Channel 1, in other words, when the channel pay switch 4-1 and the channel selecting switch 10-1 are turned "on", the ground voltage or a voltage of level "L" appears at the pay control output terminal 5. When any other channel is selected, a voltage connected to the power supply terminal 7 appears at the pay control output terminal 5 at all times because no jumper lead is connected between the jumper lead terminals. That is, the pay control output terminal 5 is made level "H". Needless to say, a signal with a scrambled picture is sent to the contractor in the CATV, and it cannot be received by ordinary receivers. In order to restore the scrambled picture signal, a descrambler circuit not shown needs to be operated. The contractor can receive the signal restored by the descrambler function. The descrambler circuit operates on the basis of the differences "H" and "L" of the voltage level appearing at the pay control output terminal 5. More specifically, when the level is "L", the descrambler circuit connected to the pay control output terminal 5 operates to discramble the picture. On the other hand, owing to the channel selecting switch 10-1 interlocked with the channel pay switch 4-1, a voltage divided by the tuning voltage generating potentiometer 9-1 appears at the tuning voltage output terminal 12. Then, Channel 1 is selected.
Usually, in CATV receivers, a multi-throw switch is employed as a selector switch in order to facilitate the selection of a particular channel. In many of the multi-throw switches, structures are provided for preventing two or more switches from being locked simultaneously, but these are often not satisfactory on account of technical difficulties and a restriction in cost. Therefore, the descrambler circuit may often be tapped to operate an uncontracted channel without paying a contract fee comparatively easily. An example of the tapping will now be explained. Let's consider a case where, as shown in FIG. 2, Channel 1 (CH1) and Channel 3 (CH3) have been contracted for and the jumper leads 3-1 and 3-3 have been placed between the respective terminals, but the subscriber has not contracted for Channel 2. The tapping of Channel 2 in this case will be explained. In the figure, numerals 1 to 12 correspond to those in FIG. 1. Since, in this case, Channel 3 is the contracted channel, the jumper lead 3-3 is connected between the jumper lead terminals 2-A3 and 2-B3.
As described above, the typical multi-throw switch forming the channel selecting switch is not satisfactory in preventing switches from being actuated simultaneously. Therefore , when Channel 1 and Channel 3 are simultaneously selected and locked, the channel pay switches 4-1 and 4-3 and the channel selecting switches 10-1 and 10-3 are simultaneously turned "on" as illustrated in FIG. 2. Thus, the pay control output terminal 5 assumes the ground potential or "L" level, and the reception contract selecting portion 1 becomes the reception mode. On the other hand, the tuning voltage output to terminal 12 is the divided voltage intermediate the voltages of the tuning voltage generating potentiometers 9-1 and 9-3. Since CATV broadcasting frequencies are arranged at substantially equal intervals, the intermediate divided voltage appearing at the tuning voltage output terminal 12 becomes approximately equal to a tuning voltage output for receiving Channel 2. By simultaneously locking the channel selecting switches of Channel 1 and Channel 3, therefore, it is permitted to normally operate the descrambler circuit for the uncontracted channel 2. In this manner, the prior-art channel selector has the disadvantage that the uncontracted channel signal can be tapped.
In another prior-art television channel selector, a reception contract selecting portion 1 is constructed conversely to that in FIG. 1. That is, jumper leads are connected to all channels in advance, the jumper leads are disconnected for contracted channels, and descrambler circuits are operated when the "H" level appears at a pay control output terminal 5. Also in this case, the same disadvantage as above described is involved.